With existing aqueous batteries such as nickel hydrogen and lead batteries, the cell unit voltage has been limited to about 1.2 V in view of the restriction in respect of the electrolytic voltage of water. In recent years, however, the downsizing of mobile devices and effective utilization of natural power generation are needed and thus, a necessity of lithium ion cells, with which a higher voltage is obtained and which is high in energy density, is being increased. For the packaging material used for such lithium ion cells, metal cans have hitherto been in frequent use. Requirements have now been made in respect of the thinning and diversification of products to be applied, under which laminate packaging materials shaped in the form of pouches made of laminates of resin films on an aluminum foil are being widely used because of their low fabrication costs and the like.
A secondary cell formed of a combination of a power generation element, such as a power collector, wherein charge and discharge are performed, and such a laminate packaging material as mentioned above may also be called cell pack or battery cell.
In FIG. 7, an ordinary cell pack is shown as a perspective view. This cell pack 150 includes a cell body 151 wherein a power generation element, not shown, is tightly sealed with a laminate packaging material (hereinafter referred to as “body packaging material”) 152, and tabs 153 each serving as an electrode terminal to which electric power is supplied from the cell body 151.
FIG. 8 is a perspective view showing the tab 153. The tab 153 has a lead 154 serving as a metal terminal, and a terminal covering resin film 155 for secondary cell covering part of an outer peripheral surface of the lead 154. The terminal covering resin film for secondary cell may also be called “tab sealant”, for which the terminal covering resin film for secondary cell may be sometimes referred to as “tab sealant” hereinafter in the present specification.
In the tab 153, the lead 154 has a first end portion 154A electrically connected to a power generation element inside the body packaging material 152, and a second end portion 154B which is electrically conductive and which is exposed at least a part of the outer surface thereof so as to permit connection with an external device, to which electric power is to be supplied. In the tab sealant 155, at least a part of the end portion 154A is covered with the body packaging material 152.
The tab sealant should have several characteristics.
The first is adhesion between the tab sealant, and the lead and body packaging material. If a space is established between the tab sealant and the lead, or between the tab sealant and the body packaging material, liquid leakage or peeling off may occur at the time of or after fabrication of a cell pack.
The second is to secure insulation. Since the lead is a current output terminal from the cell and should be covered with the tab sealant to keep insulation between the lead and other members.
More particularly, the tab sealant should preferably be excellent in securing the adhesion and insulation between the tab sealant and the lead, and should also preferably be one that tightly covers the periphery of the lead and is good in sealing properties.
To satisfy the above characteristics, a three-layer structure film is bonded to a lead through an acid-modified polypropylene in Patent Literature 1. In Patent Literature 2, an intermediate layer of a three-layer structured tab sealant is made high in melting point to secure insulation, and the adhesion between the tab sealant, and a lead terminal and a packaging material is intended to be secured by means of a relatively low melting surface layer at opposite sides along the thickness.